LBL scientists put the chill in a drill to clean up ground waste

Nitrogen gas escapes from a borehole created by the cryogenic drill.
Photo print available upon request.

May 10, 1994

By Mike Wooldridge, MAWooldridge@lbl.gov

BERKELEY -- Researchers at Lawrence Berkeley Laboratory have
put a cool twist on the underground drilling used to clean up toxic
waste sites. They have invented a drilling method that blasts
super-cold nitrogen gas as it bores, creating frozen holes that
won't collapse even in the sandiest of soils.

Nitrogen is injected down the drill's center pipe and exits
through nozzles near the spinning drill bit. At -196 degrees C,
the gas freezes difficult soils rich in sand, gravel or ash long
enough for workers to insert stabilizing metal casings into the
holes before the ground thaws.

The cryogenic method should be valuable to researchers trying
to clean up contaminated grounds at many industrial facilities,
closing military bases, and Department of Energy weapons
laboratories, where loose dirt has often made it difficult to drill
holes that do not cave in.

George Cooper, a researcher in the Earth Sciences Division at
LBL and professor of petroleum engineering at the University of
California at Berkeley, developed the technology with Rafael Simon,
a UC Berkeley graduate student.

Boring holes in the earth is usually the first step in
assessing contaminated areas. Soil samples taken from the holes
tell scientists the types of pollutants that are present as well as
their distribution. Drilling is also critical for removing
contaminants from the ground, which can involve pumping pollutants
out with groundwater or boiling them out with injected steam.

Unfortunately, cleanup experts at many facilities often find
themselves faced with crumbling soils that will not hold their
holes. "It's also no coincidence that you find difficult soils at
weapons labs -- they were built on land of little commercial
value," Cooper says. "In the west this means sandy deserts. In the
east this means agriculturally poor land, which often has sandy
soil."

Drillers at contaminated sites are at a particular
disadvantage since they can't use substances such as "drilling
muds," the clay-based goo that is used to help stabilize bore
holes. Drilling muds can complicate the cleanup effort by changing
the permeability of the soil or spreading the contaminants
further.

One location where soil collapse has been an issue is the
nuclear cleanup site in Hanford, Washington, which sits on layers
of loose gravel and sand. Cave-ins have hampered attempts to
analyze the soil beneath storage tanks that were used to store
radioactive waste.

"People have had difficulty drilling vertical holes at
Hanford, and those are the easy ones," Cooper says. "To get to the
soil beneath the tanks, you need to drill holes at an angle. These
are even less stable."

The cryogenic drilling should allow scientists to drill the
difficult holes at Hanford and other problem sites such as the
Savannah River site in South Carolina, Idaho National Engineering
Laboratory, and the Oak Ridge Reservation in Tennessee.

The method performs well in other problem soils as well, such
as the clay-rich dirts that clog up drill bits. Clay soil is
frozen hard by the nitrogen gas and is crushed by the drill bit
into rock-hard chips, which are blown out of the hole by the gas
pressure.

"The nitrogen gas also has an advantage when it comes to
sampling soil for monitoring since the freezing locks the
pollutants in," Cooper says. "Methods that use water may wash some
of the pollutants completely away. You may seriously underestimate
the level of pollution."

Engineers have occasionally used cryogenic techniques in the
past when working with loose soil. Construction crews may pipe in
liquid nitrogen to freeze and stabilize the surrounding ground when
working underground near buildings. This is the first time such a
super-cold idea has been put into practice with drilling.

Lawrence Berkeley Laboratory is a U.S. Department of Energy
national laboratory located in Berkeley, California. It conducts
unclassified scientific research and is managed by the University
of California.